The present invention relates to optical disk substrates for optical disk media of the type used in various optical disk devices, such as read-only type optical disks (e.g. CD-Audio, CD-I, CD-ROM, Video-CD, LD, DVD-Video, DVD-ROM, etc.), write-once (WORM) type optical disks (e.g. CD-R, DVD-R, etc.), re-writable type optical disks (e.g. DVD-RAM, DVD-RW, MO, etc.); and, the invention also relates to a method of manufacturing such optical disk substrates and to an apparatus for manufacturing such optical disk substrates.
The conventional method of duplicating an optical disk comprises the steps of: manufacturing a metal stamper having an uneven pattern (i.e. a pattern with microscopic holes, or pits, and lands on the surface of a plastic disk, hereinafter referred to as a xe2x80x9cpits-and-lands patternxe2x80x9d) consisting of light spot tracking-guide grooves and/or emboss-pits, such as address pits and pits for recording information on the surface thereof, through nickel electric plating from a photoresist-coated master; injecting a plastic substrate material melted at a raised temperature into a mold in which the stamper is placed; and cooling and taking out the substrate so molded, so that a plastic substrate on whose surface the pits-and-lands pattern has been duplicated is produced. This is a common technique (injection molding method) in the manufacturing of plastic substrates for DVD-ROM, DVD-R, DVD-RAM, DVD-RW, MO, etc. as well as currently-used CD-Audio, CD-R, CD-ROM.
In accordance with conventional technology, the stamper, onto which the pits-and-lands pattern is transferred, is manufactured through nickel electric plating from the pits-and-lands pattern formed on a photoresist layer of a photoresist master that is provided in the form of a photoresist-coated glass substrate. In this case, fine irregularities (average roughness being approximately 1 to 3 nm or so) generated, irrespective of manufacturing objectives, on the surface of the photoresist film are also transferred onto the surface of the stamper together with the pits-and-lands pattern consisting of the light spot tracking-guide grooves and/or the address pits, or the emboss-pits, such as the pits for recording information etc. The pits-and-lands pattern is transferred onto the substrate surface from this stamper by the injection molding method in accordance with the conventional technology. Further, a reflective layer or a recording layer is formed on this substrate.
In general, information in the optical disk is reproduced by measuring the intensity of reflected light produced by light irradiated on the reflective layer formed on the substrate surface, or information is recorded in the optical disk and reproduced therefrom by forming a recording layer that changes its property physically or chemically in response to the irradiation of light thereon, using light irradiating a tracking-guide groove part of the recording layer as a guide. That is, in the latter case, the recording and reproduction of information is achieved in such a way that the recording layer formed on the substrate surface is irradiated by laser light to effect a change in the reluctance, thereof, etc. and the strength of the reflected light is used as a carrier of information. In this case, there is a problem in that, since the shape of the recording layer formed on the substrate surface takes a film shape that reflects fine irregularities generated in the surface, irrespective of the manufacturing objectives, it becomes a cause of noise and deteriorates the recording and reproduction characteristics.
The object of the present invention is to provide an optical disk substrate having improved recording and reproduction characteristics by decreasing the fine irregularities which are accidentally generated on the surface of the substrate, thereby decreasing the noise of the recording medium formed through a stacking process.
The object is achieved by irradiating with ultraviolet light (hereinafter referred to as xe2x80x9cUV-lightxe2x80x9d) the photoresist-coated master having the pits-and-lands pattern or an optical disk substrate onto which the pits-and-lands pattern was transferred.
It is generally known that the surfaces of some kinds of plastics are modified by the irradiation of UV-light having a short wavelength and hence a high energy. This is due to a mechanism whereby the UV- light irradiation cuts stable chemical bonds existing on the surface of the plastic and oxygen atoms in the air connect to disconnected bonds or the like.
Since the optical disk substrate manufactured by injection molding is made of a plastic material, the irradiation of UV-light having a short wavelength and hence a high energy cuts the chemical bonds in a thickness range from the substrate surface to the depth of only a few gm, which in turn induces an incrementing of the light absorption coefficient in a wavelength region from 280 to 400 nm and also causes the substrate surface to be flattened to a smooth surface. In this case, when the main wavelength of the irradiating UV-light is 254 nm, which generates no ozone, decomposition in the vicinity of the substrate surface and smoothing of the surface occur significantly; whereas, if UV-light having a 184 nm wavelength that generates ozone is used together with the UV-light having a 254 nm wavelength, ozone is generated by this UV-light in the vicinity of the light source, and, in turn, the UV-light having a 254 nm wavelength is absorbed by a decomposition reaction of ozone. Consequently, an increase in the optical absorption and a smoothing of the substrate surface in the vicinity of the substrate are difficult to achieve.
In the case where the optical disk substrate is polycarbonate, which is a plastic containing oxygen atoms on the main organic atomic-chain thereof, the effect produced by the UV-light irradiation becomes significant.
Therefore, in accordance with the present invention:
(1) the substrate to be used is an optical disk substrate made of polycarbonate that has a pits-and-lands pattern consisting of light spot tracking-guide grooves and/or address pits, or emboss-pits such as pits for recording information etc., characterized in that the transmittance of the substrate is not more than 50% at one wavelength in a wavelength region from 300 to 375 nm. In a substrate that has not been treated by UV-light irradiation, because the transmittance at this wavelength is much higher than that of the substrate of the present invention, whether or not the substrate has been treated with the UV-light irradiation, its irradiation conditions, etc. can be estimated by means of simple optical measurement; and, this data can be used as an index of the surface smoothing. With another method for the purpose of smoothing the surface, the change in the transmittance differs from that of the present method. By the way, the substrate thickness to be used is specified to be 0.6 mm or 1.2 mm.
(2) The substrate to be used is an optical disk substrate as described in paragraph (1), characterized in that the wavelength dependence of the transmittance of the substrate is such that the absorption starts to increase at a wavelength of 650 nm or so and increases further toward a short wavelength side; that is, the transmittance starts to decrease, and at a wavelength of 260 nm or so and thereafter the transmittance becomes virtually 0%.
(3) The substrate to be used is an optical disk substrate as described in paragraph (1), characterized in that the hardness of the substrate in the thickness range from the surface thereof to a depth of approximately 0. 5 xcexcm is higher than that of the substrate excluding surface parts from the surface thereof to a depth of approximately 100 xcexcm by 50 to 85%. Now, since it has been confirmed that UV-light irradiation will increase the hardness of the surface, the measured hardness was used as an index of surface smoothness, which is relatively difficult to measure. If the surface hardness is high, it brings about an effect that the recording medium is hard to deteriorate even when overwriting is conducted a number of times.
(4) The substrate to be used is an optical disk substrate as described in paragraph (3), characterized in that the hardness of the substrate in the thickness range from the surface thereof to the depth of 0.5 xcexcm is not less than 140N/mm2.
(5) The substrate to be used is an optical disk substrate as described in paragraph (1), characterized in that the average roughness (Ra) of the fine irregularities generated, irrespective of the manufacturing objectives, on the substrate surface is not more than 0.8 nm.
(6) The substrate to be used is an optical disk substrate characterized in that the polystyrene equivalent weight-average molecular weight of the optical disk substrate, which has a pits-and-lands pattern consisting of light spot tracking-guide grooves and/or address pits, or emboss-pits such as the pits of recording information etc., in the thickness range from the surface thereof to a depth of approximately 20 xcexcm is smaller than that of the substrate excluding the surface parts from the surface thereof to a depth of approximately 100 xcexcm by 4 to 22%. Since it was confirmed that by irradiation of UV-light having a short wavelength and a high energy the chemical bonds on the substrate surface were cut and, consequently, the average molecular weight changed, an index of the surface smoothness can be estimated by measuring both a molecular weight histogram of the substrate in the thickness range from the surface thereof to a depth of approximately 20 xcexcm and a molecular weight histogram of the substrate excluding the surface parts from the surface to a depth of approximately 100 xcexcm and comparing these histograms.
(7) The optical substrate to be used is an optical disk substrate described in the paragraph (6) characterized in that the polystyrene equivalent average molecular weight of the optical disk substrate in the thickness range from the surface thereof to a depth of approximately 20 xcexcm is not more than 3.0.
(8) The optical substrate to be used is an optical disk substrate as described in one of the paragraphs (1) to (6), characterized in that the substrate is made of a plastic containing oxygen atoms on the main organic atomic-chain thereof as a main component. Due to UV-light irradiation at a short wavelength and a higher energy, the substrate is caused to undergo an optical reaction (light induced Fries rearrangement). It was found that, during this reaction, the main organic atomic-chain containing oxygen atoms was cut and the substrate surface was modified so as to be smoother. Because of this mechanism, the effect of UV-light irradiation can be estimated beforehand from knowledge of the main component of the plastic used.
(9) The information recording medium to be used is an information recording medium characterized in that an optical disk substrate selected from the group consisting of the substrates of the paragraphs (1) to (6) is used, and a reflective layer or a recording layer that changes in response to laser light irradiation is formed on the substrate directly or with an intermediate layer interposed. By the way, in accordance with this invention, the shape of the unevenness on the optical disk substrate is called a light spot guiding groove (so-called groove), and it is assumed that information is recorded on the recording layer on the light spot guiding groove. However, a region where information is recorded is not necessarily a groove in particular. The fundamental idea of the present invention is to flatten minute irregularities in the surface shape by UV-light irradiation, which irregularities are generated at the time of forming the shape of the unevenness on the optical disk substrate and cause a noise component in the signal. Therefore, the present invention is effective, for example, for the case where the information recording region is a region between the grooves (so-called land). Especially, in a scheme where information is recorded both on the land and on the groove (land-groove scheme), the depth of the trench (equivalent to the size of the step between the land and the groove) is xcex/6n (xcex: wavelength of a laser beam used for reproducing information, n: refractive index of the optical disk substrate at a wavelength xcex) and is deep compared to a trench depth of xcex/8n in the case of the groove recording scheme. When the trench is deep, as in this case, generally the noise component tends to become larger. However, the use of the present invention makes it possible to reduce the noise of the optical disk substrate for a land-groove recording scheme significantly.
Although it becomes possible for noise reduction of the optical disk substrate to be achieved by the method described above in detail, when the optical disk substrates are manufactured in mass production, various problems tend to occur. For example, the time necessary for manufacturing one sheet of the optical disk substrate by a normal injection molding machine is a few seconds to 10 seconds or so. However, in the case where the method of manufacturing the optical disk substrate according to the present invention was adopted, the time necessary for manufacturing one sheet of the optical disk substrate increased to a few hundred seconds, and, hence, the method was not practical. Moreover, when a few tens of thousands of sheets of the optical substrates were irradiated by UV-light, since the output of the ultraviolet lamp decreased gradually, there arose a problem that a sufficient noise reduction effect was not achieved over the course of time. With an intent to solve these problems, the present inventors carried out an extensive investigation and found that these problems can be solved by the method of manufacturing an optical disk substrate and the apparatus for manufacturing an optical disk substrate that will be described below.
(10) A method of manufacturing an optical disk substrate, comprises a step of reforming the surface of the above-stated polycarbonate by causing an oxygen containing gas to flow on the optical disk substrate, which is composed of a plastic material containing oxygen atoms in the main organic atomic-chain thereof and has irregularities on its surface, and by irradiating ultraviolet light having a wavelength of approximately 254 nm on the optical disk substrate while shielding light having a wavelength of approximately 185 nm.
(11) An apparatus for manufacturing an optical disk substrate that performs surface processing of the optical disk substrate, comprises a UV-light source for irradiating light having a wavelength of approximately 254 nm while shielding light having a wavelength of approximately 185 nm, and means for changing the distance between the optical disk substrate and the UV-light source.
(12) An apparatus for manufacturing an optical disk substrate that performs surface processing of the optical disk substrate, comprises an UV-light source for irradiating light having a wavelength of approximately 254 nm while shielding light having a wavelength of approximately 185 nm, and means for the optical disk substrate and the UV-light source.
(13) An apparatus for manufacturing an optical disk substrate that performs surface processing of the optical disk substrate, comprises an UV-light source for irradiating light having a wavelength of approximately 254 nm while shielding light having a wavelength of approximately 185 nm, and means for controlling the time of the UV-light irradiation from the UV-light source onto the optical disk substrate.
(14) An apparatus for manufacturing an optical disk substrate that performs surface processing of the optical disk substrate, comprises a plurality of UV-light sources for irradiating light having a wavelength of approximately 254 nm while shielding light having a wavelength of approximately 185 mn, and means for controlling the emission energy of each of the UV-light sources independently.
Moreover, there arose a problem in that, depending on the material and shape of the substrate holder for holding the optical disk substrate during the UV-light irradiation, the optical,disk substrate is heated, and, consequently, the optical disk substrate deformed substantially. To solve this problem, all that is needed is to use an apparatus for manufacturing an optical disk substrate as described below.
(15) An apparatus for manufacturing an optical disk substrate according to paragraph (12), comprising a substrate holder made of a fluoroplastic for holding the optical disk substrate.
(16) An apparatus for manufacturing an optical disk substrate according to paragraph (15), wherein the fluoroplastic is polytetrafluoroethylene.